It is known in the prior art to monitor a parameter associated with the charging condition of battery packs in order to terminate charging as soon as a full level of charge is reached. This prevents battery packs from being damaged and thus prolongs their service life. Continued charging of the battery pack beyond the full charging level may have serious disadvantages for all types of batteries, especially the nickel-cadmium batteries.
A number of different systems are known in the art to detect full battery charge in a charging system. One method of determining full charge is by monitoring the temperature of the battery pack. These types of systems, however, suffer the drawbacks of repeated repetition of high temperature, low charging efficiency, and problems with safety in defective cells. A second type of charging system uses a voltage cutoff technique. These types of systems have proved to be unsatisfactory in that temperature variations lead to large voltage variations, and thus, an inaccurate full charge determination. Another type of charging system incorporates the termination of the charging as a function of the time of charging. These types of systems have been unreliable in that it is difficult to accurately tell what the state of the charge of the battery pack is at the initiation of the charging sequence.
A more reliable method of charging has been disclosed in which the charging device monitors the slope of the voltage-time curve for a particular battery. Since the voltage-time charging curve for a particular battery will always be substantially the same, it is possible to determine different points on the curve which represent different points in the charging sequence, and thus it is possible to determine which point of the curve represents full charge.
A quick charging system incorporating a type of slope monitoring technique is disclosed in U.S. Pat. Nos. 4,388,582 and 4,392,101, both to Saar et al. The Saar et al. patents disclose a quick charging technique which analyzes the charging of a battery by noting inflection points which occur in the curve as the electrochemical potential within the battery changes with respect to time. By determining specific inflection points in the charging curve, it is possible to accurately terminate the rapid charging when the battery receives full charge.
The inflection point type analysis can be illustrated by viewing FIG. 1. FIG. 1 is a typical voltage-time curve of a nickel-cadmium ("Ni-Cad") battery. As is apparent, the voltage continuously rises as the charging time increases until it gets to a maximum charge point. Although the specific values of the curve may differ from battery to battery, the general shape of the curve is typical for all nickel-cadmium batteries. Further, every type of rechargeable battery will have a voltage-time curve indicative of its type.
As is apparent, the curve can be separated into five distinct regions. Region I represents the beginning of the charging sequence. In this region, the voltage characteristics are somewhat unreliable and may vary from battery to battery in accordance with its prior history of being charged and discharged. It is for this reason that region I is shown as a dotted line. Further, this region is not important in the charging sequence since it is generally traversed within a relatively short period of time after the start of the charging sequence.
After approximately 30 to 60 seconds of starting the charging sequence, the charging curve will enter the more stable region of region II. Region II is generally the longest region of the charging sequence, and is marked by most of the internal chemical conversion within the battery itself. As is apparent, the voltage of the battery does not increase substantially over this region. At the end of region II is an inflection point A in the curve. Inflection point A represents a transition from region II to region III and is noted by a point where the slope of the curve changes from a decreasing rate to an increasing rate.
Region III is the region in which the battery voltage increases quite rapidly. As the battery reaches its fully charged condition, the internal pressure and temperature of the battery also increase substantially. When these effects begin to take over, the increase in battery voltage begins to taper off. This is noted as the inflection point B.
Region IV represents the fully charged region between inflection point B and the peak of the curve represented by point C. The voltage only stabilizes at point C for a short period of time. If charging continues, the additional heating within the battery will cause the voltage of the battery to decrease and, in addition, may damage the battery.
By analyzing the inflection points of the voltage-time curve, it can be determined at what point the battery has reached maximum charge. This is done by first determining inflection point A and then looking for inflection point B. Once inflection point B is observed, the charging process can be discontinued. Since it is possible to determine the inflection points very readily and accurately, it is possible to halt the charging process, or maintain the charging process at a maintenance charge, following detection of the second inflection point.
A battery charging system which incorporates the above-described analysis of the voltage-time curve of a rechargeable battery is described in U.S. Pat. No. 5,352,969, assigned to the assignee of the present invention and herein incorporated by reference. The system described in the 5,352,969 patent is capable of charging batteries of different voltages on the same charger. A typical battery charger which can incorporate the charging system shown in the 5,352,969 patent is shown in U.S. Pat. No. 5,144,217. These types of battery chargers and others incorporating the voltage-time curve analysis also incorporate a temperature monitoring system for the protection of the batteries and the charging system in general. The temperature monitoring systems in general incorporate a temperature sensitive element, such as a thermistor, which must be brought into close association with or in contact with the battery cell.
One prior art technique for monitoring battery cell temperature consists of locating a thermistor during charging in a suitable recess in the battery pack in a position adjacent to one of the battery cells. This technique is somewhat inaccurate and thus unsatisfactory in practice since although the thermistor is located adjacent a battery cell, it may not be brought into contact with the battery cell. The thermistor, therefore, fails to detect the actual temperature of the battery cell, and instead detects the ambient temperature adjacent to the battery cell. The inaccurate battery cell temperature may lead to the overcharging of the battery or the battery pack.
Another version of these prior art battery charges requires the operator to position the thermistor on the battery pack at the time of charging the battery. If the operator forgets to position the thermistor or positions it incorrectly, the charging of the battery pack will continue until it is completely destroyed. Additional versions of these prior art chargers with temperature monitoring systems incorporate the thermistor in the structure of the battery pack during its manufacture. This not only increases the complexity and costs associated with each battery pack, it also requires that a correct connection be established between the battery charger and the thermistor when the battery pack is attached to the charger. An incorrect connection of the thermistor with the charger will lead to incorrect temperature information which will lead to the charging of the battery pack until it is destroyed. Moreover, in practice, it is found that the electrical connections of the thermistor or the thermistor itself deteriorate when the battery pack is in use, particularly when the battery pack is used with portable tools that vibrate.
An attempt has been made to obviate the disadvantages of these prior art battery chargers which rely on a thermistor in the structure of the battery pack by incorporating the thermistor as part of the charger and positioning the thermistor on one of the terminals of the charger which communicate with the battery pack being charged. A detection system can be incorporated into the charger such that when connection is detected between the thermistor and the rechargeable battery cell of the battery pack, the charging process is permitted. Otherwise, the charging process is prohibited.
While these later designs for battery chargers which include a thermistor on one of the charger terminals have successfully resolved some of the problems associated with charging battery packs, there are still some unresolved issues relating to these chargers. The positioning of the thermistor on the terminal of the charger only places the thermistor in relatively good thermally conductive relationship with the battery cells. The chain of this conductivity is through the terminal, to the battery pack terminal and to the battery cells themselves. Thus, the temperature being read by the thermistor is not the temperature of the battery cells. It is the temperature of the battery charger terminal which is being heated by the battery cells producing a time delay between the actual temperature of the battery cells and the temperature being sensed by the thermistor on the battery terminal.
Accordingly, what is needed is a technique for determining the actual temperature of the battery cells being charged using the output of a temperature sensing device, preferably a thermistor, which is a part of the battery charger and is preferably located on one of the terminals of the charge. This would provide the complexity and cost savings associated with having the temperature sensing element a part of the battery charger while simultaneously providing for an accurate determination of the temperature for the battery cells.